Well...... you can take a look at the IMAP functions, but that route will tend to be slower than simply having the server talk directly to sendmail (on *Nix) or your smtp-server (on Win32). The only time I've ever experinced any slow down of mail() is if a bad email is provided (or the email server's down).
If a bad email is provided, there's not much you can do. Ditto for the server problem unless it's your box. Where and when have you experinced sluggish mail() responses?
-geoff